In a nuclear reactor such as a boiling water reactor (BWR), a plurality of control rods or blades are selectively inserted into and withdrawn from a reactor core containing nuclear fuel bundles therein. The fuel bundles are disposed in flow or fuel channels which are spaced apart from each other to define passages through which the control rods may be translated upwardly or downwardly. Exemplary control rods have cruciform cross sections which are disposed in complementary shaped passages between adjacent fuel bundles. As the control rods are translated upwardly and downwardly, they intermittently slide against the fuel channels.
In order to reduce abrasion between the sliding control rods and the fuel channels, the control rods typically include pin-mounted rollers which provide the sole points of contact between the control rods and the fuel channels as the control rods are translated. In order to reduce wear of the roller and pins themselves, they are typically formed from conventional abrasion resistant materials containing cobalt. However, it has been determined through operation of the reactors, that the neutron flux within the reactor core irradiates the rollers and pins causing them to become highly radioactive. As the rollers and pins wear and corrode during operation, the particles released thereby remain radioactive and are circulated along with the reactor water channeled through the core. These radioactive particles increase the radiation levels of plant equipment through which the water is circulated.
Accordingly, the cobalt content of rollers and pins is being reduced or eliminated in newer reactors to reduce or eliminate the increased radiation due to the radioactive cobalt particles. However, cobalt-containing rollers and pins are presently in service in nuclear reactors, and the replacement of the entire control rod including the cobalt-containing rollers and pins joined thereto would appear to be impractical and costly.
In order to take advantage of the remaining life of control rods containing the irradiated rollers and pins, machines are presently being considered for removing the rollers and pins remotely underwater without damaging the control rod itself, and then replacing the roller with a new non-cobalt containing roller, or with a guide or spacer pad instead. A spacer pad includes a smooth head which acts as a sliding surface against the fuel channel and is secured through the aperture in the control rod which previously contained the original roller. In one conventional embodiment, the spacer pad includes two mating parts each having a head disposed on opposite sides of the control rod panel and complementary shanks which are swaged or cold rolled together for securely joining the parts together and to the panel. However, cold forming underwater is a difficult process and plastically deforms the shanks which affects their useful lifetime in the reactor.
Another conventional spacer pad has a shank with external threads which is threaded into the panel aperture for securing the spacer pad directly to the panel, and is additionally tack welded thereto to prevent the removal of the spacer pad therefrom. Installation of this assembly underwater is also difficult, and the welding process leaves residual stresses which can also adversely affect the useful life of the spacer pad and control rod.